1. Field of the Invention
This invention relates to a method of and apparatus for thermally reclaiming a material, such as foundry sand.
2. Background Description
Used foundry sand is normally subjected to reclamation so that it can be re-used in foundry processes. Such reclamation can take the form of mechanical attrition, whereby the sand is broken down into grain-size particles. However, used foundry sand contains a high proportion of chemical bonding agents, notably phenolic resins, and after a while these agents reach such a level that the sand becomes unusable, even with mechanical reclamation. Consequently, there is a need for a thermal reclamation technique whereby the chemical agents are incinerated, leaving relatively clean sand ready for re-use. Such reclamation is typically conducted in a furnace having a fluidized bed.
Because the chemical agents are very volatile, in principle, the reaction in the fluidized bed is substantially self-sustaining. That is to say, in theory at least, once the combustion process has reached a steady state from start-up (typically at around 800.degree. C.), there is no need to supply significant amounts of fuel gas since the combustion is supported instead by burning of the chemical agents. In practice, however, this does not happen for the following reasons.
According to conventional practices, the sand/bonding agent mixture is fed into the fluidized bed from above and will comprise a mixture of relatively light particles (i.e., dust) and relatively heavy particles. The velocity of the air passing through the fluidized bed (which must be above the minimum required to maintain fluidity) is such that it exceeds the settlement rate of the relatively light particles and carries these light particles upwardly into the hood and stack of the furnace. This effect is compounded by expansion of the air as it is heated by the bed. Because the relatively light particles tend to comprise a high proportion of the bonding agent (up to 50% in some cases), they have a high calorific value and will burn in the hood or stack, subject to there being sufficient oxygen available. This causes excessive heat generation in the upper parts of the furnace.
The relatively heavy particles are also highly volatile, with the result that ignition tends to occur spontaneously as the particles impinge upon the top of the fluidized bed, so that bulk of the combustion takes place in the top region of the bed. This not only acts against the supposed self-sustaining reaction of the combustion, but also adds to the heating effect on the hood and stack. These combined effects give rise to excessive heat in the flue gases and, indeed, it is sometimes the case that the flue gases end up hotter than the fluidized bed itself.
In order to deal with this problem, attempts have been made to recover some of the heat from the flue gases by using a heat exchanger. The recovered heat is then used either to pre-heat the incoming sand/bonding agent mixture or to pre-heat the air supplied to the fluidized bed. However, this gives rise to further problems. For example, pre-heating of the sand/bonding agent mixture can make the sand sticky and unmanageable and can also give rise to the emission of noxious vapors. Also, the amount of heat recoverable from the hot flue gases far exceeds that needed for pre-heating the air for the fluidized bed. Furthermore, heat exchangers are high maintenance items and the dust entrained in the flue gases tends to adhere to the heat-exchange surfaces, causing a build-up which must be cleared periodically.
Another problem arises due to the fact that, when feeding from above, it is very difficult to do anything other than put the sand/bonding agent mixture into a very localized region of the fluidized bed. This creates a cold spot at the point of entry of the mixture into the bed. Indeed, it is often the case that this spot cools to such an extent that the bonding agent ceases to be incinerated satisfactorily, so that noxious vapors containing a high proportion of unburnt hydrocarbons are given off. For this reason, it is often necessary to use an afterburner.
Furthermore, because the sand/bonding agent mixture is fed into the bed from above, the silos or hoppers containing the mixture must be provided at an elevated location for gravity feed of the material into the bed. This in itself gives rise to a degree of inconvenience.